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          <h1 class="post-title" itemprop="name headline">Const</h1>
        

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        <h4 id="const"><a href="#const" class="headerlink" title="const"></a>const<a id="more"></a></h4><figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="meta-keyword">define</span> kW 414</span></span><br><span class="line"><span class="comment">//宏是没法改的 kW = 10; 也就是 414 = 10;这是不可能的</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">const</span> CGFloat kWidht = <span class="number">414</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//const 不能修改的,加上const可以成为全局常亮</span></span><br><span class="line"><span class="comment">//两者区别: </span></span><br><span class="line"><span class="comment">//使用宏定义时候,在某一处用到时会产生临时内存占用,当然用完会释放</span></span><br><span class="line"><span class="comment">//使用const时不会产生多个临时内存占用,他只有一块内存</span></span><br><span class="line"><span class="comment">//苹果推荐使用const,为了避免在别的文件中引用 const前面加个static</span></span><br></pre></td></tr></table></figure>
<h5 id="使用细节"><a href="#使用细节" class="headerlink" title="使用细节"></a>使用细节</h5><figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//引用的时候加const告诉他这是个常亮不能改</span></span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">const</span> CGFloat kW;	</span><br><span class="line"><span class="comment">//这样会直接报错,不能编译通过</span></span><br><span class="line">kW = <span class="number">10.0</span>	 </span><br><span class="line"><span class="comment">//这段代码放在上面也可以</span></span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">const</span> CGFloat kW;</span><br></pre></td></tr></table></figure>
<h5 id="const常亮利用extern用到很多文件使用细节"><a href="#const常亮利用extern用到很多文件使用细节" class="headerlink" title="const常亮利用extern用到很多文件使用细节"></a>const常亮利用extern用到很多文件使用细节</h5><figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//创建一个.H文件,文件中extern const常亮</span></span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">const</span> CGFloat kW;</span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">const</span> CGFloat kH;</span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">const</span> CGFloat kX;</span><br><span class="line"><span class="keyword">extern</span> <span class="keyword">const</span> CGFloat kY;</span><br><span class="line"></span><br><span class="line"><span class="comment">//也可以学习苹果的写法UIKIT_EXTERN这个是跟编译环境判断的(一样的)</span></span><br><span class="line">UIKIT_EXTERN <span class="keyword">const</span> CGFloat kY;</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">//在创建一个.M文件定义const常亮</span></span><br><span class="line"><span class="keyword">const</span> CGFloat kW = <span class="number">414</span>;</span><br><span class="line"><span class="keyword">const</span> CGFloat kH = <span class="number">736</span>;</span><br><span class="line"><span class="keyword">const</span> CGFloat kX = <span class="number">414</span> / <span class="number">2</span>;</span><br><span class="line"><span class="keyword">const</span> CGFloat kY = <span class="number">736</span> / <span class="number">2</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//然后在用到的地方直接导入.H头文件即可,也可以将.H文件放在PCH中</span></span><br></pre></td></tr></table></figure>
<h5 id="添加const的位置"><a href="#添加const的位置" class="headerlink" title="添加const的位置"></a>添加const的位置</h5><figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> <span class="keyword">int</span> *p;</span><br><span class="line"><span class="keyword">int</span> <span class="keyword">const</span> *p;</span><br><span class="line"><span class="keyword">int</span> * <span class="keyword">const</span> p;</span><br><span class="line"></span><br><span class="line"><span class="comment">//首先了解一下指针</span></span><br><span class="line"><span class="keyword">int</span> a = <span class="number">10</span>;</span><br><span class="line"><span class="keyword">int</span> *p = <span class="literal">NULL</span>;</span><br><span class="line">p = &amp;a;		<span class="comment">//修改p有两种 一种是 p = &amp;a; 第二种是 *p = 20;</span></span><br><span class="line">*p = <span class="number">20</span>;</span><br><span class="line">NSLog(@<span class="string">"%d"</span>,a); <span class="number">20</span></span><br><span class="line">  </span><br><span class="line"></span><br><span class="line"><span class="comment">//结论 const右边是什么,那就是右边那种方式不能修改</span></span><br><span class="line"><span class="comment">//比如下边const右边是*p, 那么  *p = 20; 是不能通过编译的,而 p = &amp;a;是可以编译通过的</span></span><br><span class="line"><span class="keyword">const</span> <span class="keyword">int</span> *p;</span><br><span class="line"></span><br><span class="line"><span class="comment">//所以下面两行代码作用是一样的</span></span><br><span class="line"><span class="keyword">const</span> <span class="keyword">int</span> *p1;</span><br><span class="line"><span class="keyword">int</span> <span class="keyword">const</span> *p2;</span><br><span class="line"></span><br><span class="line"><span class="comment">//而int * const p; 也就意味着 p = &amp;a; 是不能通过编译的, *p = 20;是可以修改的</span></span><br></pre></td></tr></table></figure>
<h5 id="使用场景"><a href="#使用场景" class="headerlink" title="使用场景"></a>使用场景</h5><figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">test</span> <span class="params">(<span class="keyword">const</span> <span class="keyword">int</span> *p)</span></span>&#123;</span><br><span class="line">    <span class="comment">// 意味着在这个函数中不用 *p = 100;修改外面的值 </span></span><br><span class="line">    <span class="comment">//但是可以这样修改</span></span><br><span class="line">    <span class="keyword">int</span> num1 = <span class="number">100</span>;</span><br><span class="line">    p = &amp;num1;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">main&#123;</span><br><span class="line">    <span class="keyword">int</span> num = <span class="number">20</span>;</span><br><span class="line">    test(&amp;num);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h5 id="OC中const位置"><a href="#OC中const位置" class="headerlink" title="OC中const位置"></a>OC中const位置</h5><figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//定义</span></span><br><span class="line">NSString * <span class="keyword">const</span> name = @<span class="string">"jack"</span>;</span><br><span class="line"><span class="comment">//引用</span></span><br><span class="line">UIKIT_EXTERN NSString * <span class="keyword">const</span> name;</span><br><span class="line"></span><br><span class="line"><span class="comment">//防止修改 name = @"rous";</span></span><br></pre></td></tr></table></figure>

      
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